1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by scanning a photosensitive member with a plurality of beams.
2. Description of the Related Art
For electrophotographic image forming apparatuses such as laser beam printers which form an image by scanning a photosensitive member with beams, a scanning optical system in which beams output from light-emitting devices (hereafter referred to as “LDs”) are collected by a lens system, the collected beams are deflected by a polygon mirror with rotation of a scanner motor, and the deflected beams scan the photosensitive member has been widely used.
For image forming apparatuses having this scanning optical system, the technique to form an image by scanning a plurality of beams at the same time from an increased number of LDs has been proposed so as to realize higher speed image formation and higher resolution. Particularly in a vertical cavity surface emitting laser (hereafter referred to as a “VCSEL”), light emission points can be arranged in a two-dimensional pattern, and many emission points can be arranged on one chip.
Examples of techniques relating to image forming apparatuses that form an image by scanning beams on a photosensitive member include a technique to perform auto power control (hereafter referred to as “APC”) so as to maintain the light quantity of beams on the photosensitive member constant during image formation. Methods of APC include a method in which LDs are lighted up for a predetermined time period, the light quantity of beams is detected by light quantity detecting units (PD: photodiodes) provided inside or outside the LDs, and drive current for outputting beams according to the detected quantity of emitted light is feedback-controlled.
In general, a PD detects a beam (rear light) going out from an end face of an LD opposite to a light-emitting end face of the LD from which a beam (front light) goes toward a photosensitive member. APC is performed during a time period over which a beam scans a non-image region (a time period over which a beam does not scan the photosensitive member in one scanning cycle). Because APC is performed during this time period, APC can be performed without exposing the photosensitive member to the front light.
It is difficult in terms of layout to provide the same number of PDs, which correspond to respective beams, as the number of LDs, and even if they can be laid out, this would cost much. For this reason, APC is performed by causing a single PD to receive a plurality of beams and successively lighting up a plurality of LDs. Thus, when APC is to be performed, a plurality of LDs are successively lighted up at different times during a time period over which beams scan a non-image region.
However, in a VCSEL whose LDs can be increased with ease, beams are output in a direction perpendicular to a semiconductor substrate, and no beams are output in a direction opposite to that direction. For this reason, APC is difficult in an arrangement in which PDs are placed in the same package as with an edge emitting semiconductor laser, and hence there has been a technique to separate front light using a half mirror and cause the front light to fall on PDs.
There has been disclosed a method in which pencils of light obtained by turning light output from a surface emitting laser into parallel light by a collimator lens and limiting the same by an aperture are separated by a half mirror into light going toward a photosensitive member and light going toward a PD, and APC is performed using a single PD (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2002-40350).
The method disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-40350 is a method in which front light is separated by a half mirror method, and the separated beams are caused to fall on a PD to perform APC. In this method, it is necessary to ensure the light quantity of beams going toward a photosensitive member, and thus the light quantity of beams output from LDs has to be large. Moreover, because the light quantity of beams going toward the PD cannot be large, the amplification rate of the PD has to be high, but this will deteriorate responsiveness and signal-to-noise ratio. Moreover, the response speed of the PD generally varies according to its light-receiving area, and hence even if the light-receiving area is widened to increase the quantity of incident light, the response speed of the PD will decrease, and the amount of time for APC will increase. Also, the placement of the half mirror brings about cost increase.
For this reason, there has been proposed a method in which a PD is placed on a beam scanning line, and APC is performed according to the light quantity of received beams (hereafter referred to as “scanning light APC). In this method, the light quantity of beams can be detected only at times when beams fall on the PD on the scanning line.
Moreover, an apparatus that performs scanning light APC has problems explained hereafter. To perform scanning light APC, it is necessary to generate a synchronization signal. However, an initial state in which image formation is going to be started, APC has not been performed yet, and hence it is uncertain how much drive current should be supplied to LDs to generate a synchronization signal. In a case where a synchronization signal is to be generated by supplying drive current of a predetermined value to one LD, the drive current may break the LD because it is overcurrent.